Show output from external commands as it happens

Previously, all output was sent to the frontend at once so only the last
few rows were visible.  This was a bit disconcerting when a command took
a bit of time because there was no feedback that anything was happening.
The downside to this patch is that it takes a bit longer for a command
to finish since the frontend has to do more drawing.

src/MacVim/gui_macvim.m

@@ -210,6 +210,48 @@ gui_mch_flush(void)
 }
 
 
+    void
+gui_macvim_flush(void)
+{
+    // This function counts how many times it is called and only flushes the
+    // draw queue if called sufficiently often.  The first few times it is
+    // called it will flush often, but the more it is called the less likely is
+    // it that anything will be flushed.  (The counter resets itself if the
+    // function isn't called for a second.)
+    //
+    // NOTE: Should only be used in loops where it is impossible to know how
+    // often Vim needs to flush.  It was written to handle output from external
+    // commands (see mch_call_shell() in os_unix.c).
+
+    static CFAbsoluteTime lastTime = 0;
+    static int delay = 1;
+    static int counter = 0;
+    static int scrolls = 0;
+
+    CFAbsoluteTime nowTime = CFAbsoluteTimeGetCurrent();
+    CFAbsoluteTime delta = nowTime - lastTime;
+    if (delta > 1.0)
+        delay = 1;
+
+    // We assume that each call corresponds roughly to one line out output.
+    // When one page has scrolled by we increase the delay before the next
+    // flush.
+    if (++scrolls > gui.num_rows) {
+        delay <<= 1;
+        if (delay > 0x10000)
+            delay = 0x10000;
+        scrolls = 0;
+    }
+
+    if (++counter > delay) {
+        gui_macvim_force_flush();
+        counter = 0;
+    }
+
+    lastTime = nowTime;
+}
+
+
 /* Force flush output to MacVim.  Do not call this method unless absolutely
  * necessary. */
     void

src/os_unix.c

@@ -4435,8 +4435,10 @@ mch_call_shell(cmd, options)
 			cursor_on();
 			out_flush();
 # if FEAT_GUI_MACVIM
-			if (gui.in_use)
+			if (gui.in_use) {
 			    fast_breakcheck();
+			    gui_macvim_flush();
+			}
 # endif
 			if (got_int)
 			    break;

src/proto/gui_macvim.pro

@@ -14,6 +14,8 @@ gui_mch_update(void);
     void
 gui_mch_flush(void);
     void
+gui_macvim_flush(void);
+    void
 gui_macvim_force_flush(void);
     int
 gui_mch_wait_for_chars(int wtime);